Congress has many oversight powers, some that they use more then others, powers like advice and consent on things like treaties which require a two- thirds majority in the senate. They also have oversight on executive and judicial appointments which need senate approval, congress can also override the presidential veto, only if they can muster a two thirds majority in both chambers. Congress also has impeachment powers but is rarely used.
So Congress has different oversight powers, but are not used that often, so there not effectively if there not being used, but when they used them they can be effectively. The power of advice and consent from which is where Treaties are subject to ratification and require a two thirds majority in the senate, this was used to stop the Treaty of Versailles, which concluded the First World War, one of the reasons behind the rejection is that the Treaty require the USA to join the League of Nations, which went against their view of being a isolated country.
Successive president often use executive agreements with heads of foreign governments which does not require senate ratification. This show the oversight powers of congress have been used effectively as they stop treaties from being sign that go against thinking in the USA, and which will cost them more then they will get out of it, so have forced presidents to found a different way around them from congress needing to ratification it. Congress also has oversight on executive and judicial appointments which them allows from the senate to say yes or no to the appointment.
They do this by holding confirmative hearings which are normal uncontroversial but some have become bitter battles, as in 1987 President Reagan’s nominee to the supreme Court bench Robert Bork was rejected and again in 1989 John Towers was turned down as secretary if defence. Which show that they could be doing it to stop someone they don’t want, or a person against their view, say in President is Republic but Congress is controlled by the Democratic then they would rejected someone with very Conservatives view and are right winged, but may say yes to someone you have liberal views and is more centred in political thinking.
Congress can also override the presidential veto as long as they muster a two third majority in both chambers, but this is a formidably difficult process and overrides are in practice rare. Which shows when you look at 1989 to 1998 there have been 56 to override a presidential veto, but only 2 have had the two thirds majority to override the veto. This does show that power is not used effectively as it could and that congress doesn’t use it a lot.
Congress also has impeachment powers but these are only invoked in exceptional circumstances, only two presidents Andrew Johnson and Bill Clinton have been impeached and were both acquitted at the conclusion of their trails before the senate, there was also former president Richard Nixon after Watergate would have been impeached and face criminal charges, but resign before the trail took place and was pardon but his successor, Gerald Ford.
So congress does use it powers of oversight effectively but are rare in use like impeachment and the overriding a veto because it is hard to do so they don’t use their powers and that it also that the president has found other ways around so the powers of congress are not used or are not be able to be used so are not used effectively but when they are they are used effectively.